It is known to convert polysaccharides to chemical derivatives by reactions performed on their monomeric units and to impart in this way desired properties to the polymer through altered chemical structure, for example by adding functional groups to the polymer molecule. Cellulose, which is an abundant renewable natural substance, is one example of a polymer that can be converted to many chemical derivatives. The derivatization takes place mostly by chemical reactions of the hydroxyl groups in the β-D-glucopyranose units of the polymer. By chemical derivatization the properties of the cellulose can be altered in comparison to the original chemical form while retaining the polymeric structure.
Heterocyclic nitroxyl compounds are known as catalysts that participate in the selective oxidation of C-6 hydroxyl groups of cellulose molecules to aldehydes and carboxylic acids, the corresponding oxoammonium salt being known as the active direct oxidant in the reaction series. One of these chemical oxidation catalysts known for a long time is “TEMPO”, i.e. 2,2,6,6-tetramethylpiperidinyl-1-oxy free radical. Thus, the oxidized forms of the nitroxyl radicals, N-oxoammoniumions, act as direct oxidants in the oxidation of the target cellulose molecule, whereas a main oxidant is used to bring oxygen to the reaction chain and to convert the nitroxyl compound back to the oxidized form.
It is known to oxidize primary alcohols to aldehydes and carboxylic acids through “TEMPO” by using sodium hypochlorite as the main oxidant (for example Anelli, P. L.; Biffi, C.; Montanan, F.; Quici, S.; J. Org. Chem. 1987, 52, 2559). To improve the yield in the oxidation of the alcohols to carboxylic acids, a mixture of sodium hypochlorite and sodium chlorite was also used (Zhao, M. M.; Li, J.; Mano, E.; Song, Z. J.; Tschaen, D. M.; Org. Synth. 2005, 81, 195).
It is also known procedure to catalytically oxidize cellulose in native cellulose fibers through “TEMPO” by using sodium hypochlorite as main oxidant (oxygen source) and sodium bromide as activator (Saito, T. et al.; Cellulose Nanofibers Prepared by TEMPO-Mediated Oxidation of Native Cellulose, Biomacromolecules 2007, 8, 2485-2491). The primary hydroxyl groups (C6-hydroxyl groups) of the cellulosic β-D-glucopyranose units are selectively oxidized to carboxylic groups. Some aldehyde groups are also formed from the primary hydroxyl groups. When the fibers of oxidized cellulose so obtained are disintegrated in water by applying sufficient mechanical energy, they give stable transparent dispersion of individualized cellulose fibrils of 3-5 nm in width, that is, so-called nanofibrillar cellulose.
The catalysts used in the method are expensive, but they do not change irreversibly in the reaction. Therefore, many methods are used for recovering the heterocyclic nitroxyl catalyst such as “TEMPO” from reaction mixtures after the completion of the reaction so that it could be reused. At the same time the toxic catalyst is removed from the reaction mixture which can, after possible other purification steps, be discharged as effluent.
Methods that have been used include distillation of reaction mixture filtrate and recovery of the catalyst as distillate (JP2011116865), solvent extraction of reaction mixture with an organic solvent immiscible with water (JP2011116866), addition of adsorbent to the reaction mixture and recovery of the catalyst from the adsorbent or addition of ion exchange resin to adsorb by-products (JP2009242590), extraction of the reaction mixture with supercritical CO2 (JP2010235454), or desalting the reaction mixture by electrophoresis (WO10116794).
Publication WO2005/058851 discloses a process of separation of heterocyclic nitroxyl catalyst from an acidic aqueous medium with a cation exchange resin. After being adsorbed on the cation exchange resin, the heterocyclic nitroxyl catalyst, which is in nitrosonium and/or hydroxylamine form, can be washed off or eluted.
Publication WO2005/058818 discloses separating the heterocyclic nitroxyl catalyst remaining in the reaction mixture by contacting the reaction mixture with a hydrophobic resin and removing the catalyst from the resin by eluting it with an organic solvent or a mixture of water and a water-miscible organic solvent. The heterocyclic nitroxyl catalyst is separated from the organic solvent or mixture of water and a water-miscible organic solvent by adjusting the pH of said solution to value below 4 and removing the organic solvent to obtain a residue containing the catalyst in the nitrosonium and protonated hydroxylamine form, which are non-volatile. It is preferred that the surface area of the hydrophobic resin that is used in the separation is above 380 m2/g and the porosity is above 0.5 ml/ml. Suitable types mentioned are resins available under tradename XAD, for example XAD-2, XAD-4, XAD-8, XAD-11, XAD-16, XAD-16, XAD-30, or XAD-1180.
The main problem in recovering the heterocyclic nitroxyl catalyst is that it has to be recovered from a large volume of reaction mixture, where it exists at a concentration of below 0.1%, even below 0.05%. Thus, recovery of the catalyst in concentrated reusable form with a good yield is important. If adsorbent is used, it is important to be able to extract the catalyst at a sufficiently high concentration from the adsorbent.